Reduce Completion Costs by Using Optimum Number of Perforations

Abstract

Abstract There has been a recent trend toward reducing the number of perforations in oil wells that are to be hydraulically fractured. Two benefits of this practice are better control of the fracture treatment and reduced completion costs. This paper presents a method of mathematically calculating the number of performations to be used to obtain the greatest reduction in completion costs. This optimum number of perforations is obtained by equating the perforating cost and the perforation friction cost during the fracture treatment. By reducing the number of perforations to this optimum point the completion costs are reduced. In the case of hydraulically fractured oil wells, the reduction of the number of perforations to this optimum point will have negligible effect on the production capacity of the well. Introduction Ever since the introduction of the practice of setting casing through the pay zones and then perforating, there has been the problem of determining the number of perforations that should be used. An early mathematical study was made by Muskat wherein the perforations were considered to be point sinks. The study made a comparison of the theoretical productivity possible through perforations as compared to the theoretical productivity possible in an open-hole completion. Later studies were made using electric analog models wherein the effect of penetration into the formation was considered. These studies indicated that pay zones should be perforated from top to bottom with a density of 8 to 16 shots per foot in order to not seriously restrict production. The above theoretical studies were made assuming ideal conditions and radial flow drainage with the results expressed as ratios. In actual practice many factors in addition to the number of perforations, such as proration, formation permeability, tubing choke, etc., determine the resultant productivity of a perforated well. Due to these factors, plus other things, such as perforating gun mechanical strength and interference between jet charges, the normal field practice became: perforate pay zones from top to bottom with a reduced density of four shots per foot. Statistical data comparing open-hole completions and perforated completions in one field indicated there were no recognizable differences in performance between the two groups. Recent articles have advocated the further reduction of the number of perforations in wells that are to be hydraulically fractured. This reduction in the number of perforations results in better control of the fracture treatment, and reduction in completion costs. It appears that there is a present trend in the industry to follow the suggestions in these articles.